The ability to separate a particle/fluid mixture into its separate components is desirable in many applications. Acoustophoresis is the separation of materials using sound waves, which may propagate at ultrasonic frequencies. Standing sound waves, which may have relativity high intensity, can exert forces on particles or secondary fluids in a host or primary fluid when there is a differential in density and/or compressibility, or the acoustic contrast factor. The pressure profile in an acoustic standing wave includes areas of local minimum pressure amplitudes at nodes of the waveform and local maxima at anti-nodes of the waveform. Depending on their density and compressibility, fluid or particles are urged toward and remain at the nodes or anti-nodes of the standing wave in response to the pressure profile. The higher the frequency of the standing wave, the smaller the particles that can be trapped at nodes or anti-nodes of the standing wave.
The fields of biotechnology and bioprocessing have experienced significant growth, some of which has resulted from or fostered improvements in the equipment and technology used. For example, improved equipment and techniques applied to bioreactors have allowed for larger volumes and lower cost for the production of biologically derived materials such as monoclonal antibodies and recombinant proteins. These improvements in manufacturing processes have permitted the creation of new biologically based pharmaceuticals from bioreactor processes.
A modern bioreactor tends to be a complex piece of equipment. In such equipment, a number of parameters are controlled to various degrees of specificity. For example, the bioreactor may regulate fluid flow rates, gas content, temperature, pH and/or oxygen content. All of these parameters can be tuned to allow the cell culture in the bioreactor to be efficient in producing the desired biomolecules from the bioreactor process.
There are several popular techniques for operating a bioreactor and obtaining product. Among these techniques are fed-batch, batch and perfusion processes. The perfusion process is distinguished from the fed-batch and batch processes by its lower capital cost and higher throughput.
In the fed-batch process, a culture to be grown or expanded is seeded in a bioreactor. The gradual addition of a fresh volume of selected nutrients during the growth cycle is used to improve productivity and expansion. The product, which may be, for example, a monoclonal antibody or a recombinant protein, is recovered after the culture is harvested. Separating the cells, cell debris and other waste products from the desired product may be performed using various types of traditional filters for separation. Such filters tend to be relatively expensive to manufacture and become clogged and non-functional as they retain material from the bioreactor as the material is processed. A fed-batch bioreactor process is favored because of its simplicity and also due to carryover knowledge from well-known fermentation processes. However, a fed-batch bioreactor has high start-up costs, and generally has a large volume to obtain a cost-effective amount of product at the end of the growth cycle. The processes for turning over a batch and preparing the bioreactor for a new batch often include large amounts of non-productive downtime.
A perfusion bioreactor processes a continuous supply of fresh media that is fed into the bioreactor while growth-inhibiting byproducts are continuously removed. Nonproductive downtime can be reduced or eliminated with a perfusion bioreactor process. The cell densities achieved in a perfusion culture (30-100 million cells/mL) are typically higher than for fed-batch modes (5-25 million cells/mL). These improvements have led to lower contamination in the harvest and better yields without significant increase in cost. A perfusion bioreactor uses a cell retention system to prevent escape of the culture when byproducts are being removed. The cell retention systems add a level of complexity to the perfusion process, where the process is carefully managed, controlled and maintained for successful operation. Operational issues such as malfunction or failure of the cell retention equipment has previously been a problem with perfusion bioreactors, which has limited their attractiveness in the past.
In each of the bioreactor processes, some type of separator or filter is used to separate cells, cell debris, product or byproducts from the culture media. Acoustophoresis may be used for such separation or filtering.